1. Field of the Invention
The present invention relates to a multi-layer ceramic substrate and a method for producing the same, and more particularly, to a multi-layer ceramic substrate including passive components such as capacitors and inductors, and to a method for producing the same.
2. Description of the Related Art
As electronic devices have become smaller, there have generally been used ceramic substrates made of ceramic insulator and packed with various electronic components that constitute electronic circuitry. Recently, in order to attain an increased packing density and to meet high-frequency requirements, integral multi-layer ceramic substrates have been developed. These integral substrates are made by laminating a plurality of low dielectric constant ceramic sheets, each being provided with circuit patterns by use of paste containing a low-resistance conductive material such as Ag, Ag--Pd alloy, Cu, or Au, and firing the resultant laminate.
To attain even higher packing density, it would be advantageous to form a substrate by use of a material having a low dielectric constant and to construct a structure incorporating passive components therein two-dimensionally or three-dimensionally. Particularly, three-dimensional incorporation of passive components is more favorable than two-dimensional incorporation, in view of packing density, degree of freedom in circuit design, and electrical characteristics. Achieving three-dimensional incorporation requires a so-called heterologous material-joined substrate in which the substrate material and the material for passive components are combined, the material for passive components being heterologous to the former material and including a capacitor material, inductor material, resistor material, etc. having a dielectric constant different from that of the substrate material.
To obtain such a three-dimensional passive-component inclusion structure, the following methods have conventionally been used.
A first method is called a thick-film method. The thick-film method includes the steps of printing patterns by use of dielectric paste and the like on green sheets which are to be formed into substrates by means of a thick-film forming technique, stacking the green sheets and joining them with pressure, and firing the sheets to partially include capacitors and the like in finished multi-layer ceramic substrates. This method involves the following drawbacks:
(1) Because variation in thickness of paste on the sheet is rather large and precision of printing the paste is insufficient, variation in capacitance or other characteristics is rather large.
(2) The paste on the green sheet is deformed in processes of pressure-joining the sheets and firing, which may cause variation in capacitance and like characteristics.
(3) Repeated printing and lamination of the sheets leads to deteriorated surface flatness of the printed portion, making it difficult to increase the number of sheets of the laminate or, in the case of a capacitor, to increase capacitance thereof.
A second method is described in Japanese Patent Application Laid-Open (kokai) No. 288498/1986. According to this method, chip-type ceramic passive components sintered in advance are incorporated interior of a laminate formed of a plurality of ceramic green sheets which are to be formed into a substrate. This process may overcome the drawbacks involved in the first method; however, shrinkage behavior must be strictly controlled in the X, Y, and Z directions of ceramic green sheets, and the method has another drawback in that the materials which can be used as ceramic green sheets for providing substrates are considerably limited. In addition, the method involves the following problems:
(1) When chip-type passive components are included within the space formed in a substrate, undesired gaps tend to be formed between the passive components and the corresponding inner surfaces extending perpendicular to the lamination planes, which gaps may cause formation of depressions at the surface of a substrate. Particularly, when the substrate is made of a glass-containing material, depressions become more significant, and in the worst case, cracks extending perpendicular to the lamination planes may be generated in the surface of the substrate or in the interface between the passive components and the substrate.
(2) The flatness of the substrate tends to deteriorate.
(3) Attaining a high dimensional precision is difficult.
(4) Formation of minute wiring is difficult.
As conventional methods to enable formation of high-density wiring in a multi-layer circuit substrate, the following methods are known. Japanese Patent Application Laid-Open (kokai) No. 4-243978, for example, discloses a method in which a dummy green sheet is pressure-joined onto each of the upper and lower surfaces of a laminate body to serve as a substrate including a plurality of green sheets that can be fired at low temperature, wherein the dummy green sheet does not shrink at the firing temperature of the laminate body; then the resultant pressure joined product is fired at a relatively low temperature; and subsequently the unsintered layer attributed to the dummy green sheet is removed by peeling it off after firing. Japanese Kohyo Publication No. 5-503498 discloses a modified technique of this method; pressure is further applied onto a laminate to be formed into a substrate in a direction perpendicular to the laminate planes during firing.
According to these methods, shrinkage does not occur easily in the X-Y plane, or in other words, in directions parallel to the laminate planes. Therefore, improved dimensional precision of the resultant substrate can be obtained, providing an advantage that breakage of wiring will not easily occur even when the substrate has high density wiring. However, these methods are not drawn to the case in which passive components are contained interior of a substrate.
Turning again to a method for producing multilayer circuit boards which contain passive components, a third method is disclosed in Japanese Patent Application Laid-Open (kokai) No. 9-92983. This publication discloses a method which combines the following two methods; one method being directed to prevention of the aforementioned shrinkage in the X-Y plane of a substrate, and the other method being directed to partial incorporation, into a multi-layer circuit board, of a capacitor in the form of a sheet or thin film. This method is suitable for the production of multi-layer circuit boards incorporating passive components therein and having a high density wiring.
In this third method, when the dielectric portion is made of a sheet, a dielectric layer having the same area as that of the substrate is provided. As a result, the dielectric layer is exposed in an edge face of the substrate, and thus, the dielectric layer must be dense enough not to permit penetration of moisture. To meet this requirement, pressure is applied onto the substrate from both the upper and lower surfaces thereof during firing, to thereby make the dielectric layer sufficiently dense. However, the restriction in shape of the dielectric layer causes, among others, the following drawbacks:
(1) Low level of freedom in design, due to laminar placement of the dielectric in a substrate, and
(2) Problems such as crosstalk of signals tend to occur.
In the above-mentioned third method, when the dielectric portion is formed by use of a thick film, there is sometimes included a step in which a depressed portion is formed in the sheet for substrate such that the depression corresponds to the region in which a dielectric portion is to be formed, and subsequently the depression is filled with a dielectric paste. In this case, among the problems involved in the aforementioned first method, i.e., the thin-film method, positional deviation of the thick film and variation in characteristics which may be caused by deformation of the dielectric paste during pressure-bonding of substrate sheets can be avoided; however, variation in thickness of the paste still remains, through it may become less significant. Moreover, since the dielectric portion is difficult to form into a laminate structure, there remains another problem in that large capacity cannot be easily obtained.